1. Field of the Invention
The present invention relates to a solar battery of the multilayer structure type and its manufacturing method and, more particularly, to a multilayer structure type solar battery in which energy converting efficiency was improved and a method of manufacturing such a battery.
2. Related Background Art
Hitherto, a solar battery has been used as a driving energy source in various kinds of apparatuses.
The conventional solar battery generally has a structure using a pn junction or a pin junction. FIG. 2 is a diagrammatical cross sectional view showing a cross sectional structure of a conventional solar battery having a pn junction. As a semiconductor constituting a pn junction or pin junction, Si of group IV of the periodic table, GaAs or InP as a compound of groups III and V, or the like is used. However, generally, Si is used.
In such a solar battery, hitherto, there has been much research to improve the efficiency (hereinafter, referred to as energy converting efficiency) for converting light energy into electric energy. Various examinations have been conducted.
In FIG. 2 herein, the solar battery has two semiconductor layers of opposite conductive types, namely p-type 203 and n-type 202 with a depletion region at the interface of the two layers. On the top of the battery, there is a metal grid 205 for an electrode covered with an anti-reflecting layer 204, which collects the generated carriers in the semiconductor layers when light impinges onto the surface of the battery. An electric current can be utilized when an external closed circuit is connected in front electrode 205 and back electrode 201.
For a conventional solar battery having a pin junction, the structure of the battery is slightly changed compared to that of the pn junction, that is, an intrinsic semiconductive layer with a high resistitivity is inserted between p-type and n-type layers. The structure of this type is commonly used in a solar battery which is composed of amorphous Si related materials.
The crystalline properties of a semiconductor serving as a functional portion of a solar battery are significant factors determining the energy converting efficiency. Polycrystal semiconductors are superior to amorphous semiconductors with respect to the energy converting efficiency. Monocrystal semiconductors are better than polycrystal materials in energy converting efficiency. However, the use of a monocrystalline semiconductor substrate is disadvantageous from the viewpoints of realization of a large area and low cost.
On the other hand, the structure of the solar battery is also important as a factor determining the energy converting efficiency. In the case of the solar battery having a pn junction or pin junction, by using a tandem or triple structure by laminating semiconductor layers having different band gaps, the energy converting efficiency can be improved. This is because in the tandem or triple structure using semiconductor layers having different band gaps, the light can be efficiently absorbed in a wide range from short wavelengths to long wavelengths.
However, in the solar battery having a pn junction or pin junction, as shown in FIG. 2, since the light enters perpendicularly to the pn junction surface, it is necessary that the photogenerated charge carriers diffuse in a semiconductor layer in which the lifetime of minority carriers is short due to the impurities added in the layer. Also, the solar battery is influenced by recombination at the interface between the semiconductor layer and the reflection preventing layer or metal electrode, and further, since a collecting electrode is formed on the light incident surface, the effective area of light incidence is reduced. The above mentioned factors deteriorate the energy converting efficiency. Therefore, even in the case of using an Si monocrystal, the energy converting efficiency of the solar battery as shown in FIG. 2 is at most about 15 to 18%.
As a solar battery which can cope with such structural problems, a Si solar battery of the point contact type has been proposed (R. A. Sinton, Y. Kwark, J. Y. Gan, R. M. Swanson, IEEE Electron Device Letters, Vol. EDL-7, No. 10, pages 567 to 569, October, 1986). FIG. 3 shows a diagrammatical cross sectional view of the point contact type solar battery. This type of solar battery has a substrate of high resistance semiconductor wherein several small p- and n-type areas are formed at the back surface side (they constitute pin junctions in parallel with the substrate) and has no electrode located at the front surface which is covered with a thin oxide film. Such a solar battery structure has the following advantages. A high resistance semiconductor layer when used as an active layer extends the lifetime of the minority carriers. Since the surface of the semiconductor layer is covered by an oxide film, the influence of surface recombination can be reduced. Since a pin junction is formed on the back surface side of the substrate in parallel with the substrate and point contacts are provided by the metal electrodes, the entire substrate surface is available, so that the light incidence can be effectively 100%. Further, since the surface has been textured, the light absorbing efficiency of, in particular, long wavelength light can be raised due to a light trapping effect. In a solar battery shown in FIG. 3, a high converting efficiency of 22% at AM1.5 to 27.5% under 100 suns is obtained.
However, since a solar battery with such a structure uses monocrystalline Si as the substrate material, there are drawbacks such that the manufacturing costs are high, it is necessary to make a thin film cell (thickness of 60 to 100 .mu.m) by polishing in order to reduce Auger recombination due to concentration of surface carriers, and the like. Such circumstances are similarly caused with compound semiconductors as well as with silicon.
To solve the above drawbacks, as a thin film solar battery having a large enough crystal grain diameter and a high energy converting efficiency, there has been proposed "a solar battery made of a semiconductor crystal having a mountain-shaped facet-like surface which is provided on a non-nucleating surface having a nucleating surface portion whose nucleating density is significantly larger than that of the non-nucleating surface and whose surface area is small enough to generate only a single nucleus, characterized in that the mountain-shaped facet-like surface forms a light receiving surface" (Japanese Patent Application No. 63-210358, not published before the filing date of the present invention). According to the above, a solar battery of the point contact type having a high energy converting efficiency can be cheaply provided on a non-singlecrystalline substrate.
However, according to the solar battery disclosed in Japanese Patent Application No. 63-210358, since the pin junction is located in parallel with the substrate, there is a problem that it is impossible to use a tandem or triple structure in which semiconductor layers are laminated as in the conventional solar battery of the pn junction or pin junction.